Lisa 2/5 Video Board Image Deterioration

[MacinPosh]

I recently brought my Lisa 2/5 back out of the cupboard to have a tinker during lockdown.

Unfortunately after a few minutes the contrast goes very dark and the image starts to shrink on the right hand side.

I haven't dared to leave it on long enough for something to go bang...

The BU406 is quite hot, but probably within normal limits. 

Any ideas would be much appreciated  :lisa2:

I've attached the video board schematic for reference. 

 

[bibilit]

My guess is that capacitors are failing. 

 

[MacinPosh]

A very good point, although I went through and recapped it when I first got it in 2018.

I don't think they'd wear out that quickly?

 

[bibilit]

I don’t think so. Bad solder joint or another component going bad ?

 

[stepleton]

A shot in the dark, but could this be a grounding issue? Did you remember to connect the white ground lead to the tab on the metal "cup" that shields the CRT neck? Is there a chance that this could have come loose when you moved your Lisa in or out of storage?

 

[MacinPosh]

Thanks again for the ideas.

I've had the card out a few times to inspect it and always made sure everything was connected back up, but i'll check again.

Going on the assumption that as BU406 is hot, maybe something in the Horizontal drive circuit is failing, i.e. Row A of the schematic.

I'm definitely not an expert in CRT circuitry/flybacks, if anyone has some insight into this it would be most appreciated.

I've started looking at the biasing passives but they've all checked out so far

 

[lisa2]

I have worked on dozens of these boards, and the capacitors have never been the problem for me...

Try replacing the voltage 7824CT regulator U1 ( at lower right corner of the board ), an indication that this is failing is that the the resistor R1 will be burned/cooked (and should be replaced also).  This is a very common fault on these boards as they age.

Good luck!

 

[MacinPosh]

Thanks lisa2 

I’ll replace these and report back asap. 

 

[sunder]

I've recently noticed that all of my Lisas (Lisae?) are now showing various video symptoms. At least one is a trimpot issue which should be easy to fix.

Another the display over time scrunches up at the top, but is otherwise readable and playing with the trimpots doesn't help.

Another goes out of focus. So I've also got a bunch to repair.

So it seems whatever these issues are, whether capacitor, voltage regulator, or trimpot, or other, this is roughly the lifespan of the analog video boards. I'd like to collect the symptoms and fixes, and catalog these so I can update the lisafaq with them: https://lisafaq.sunder.net/single.html#lisafaq-hw-vid_jumpy

 

[MacinPosh]

I finally got some time to replace R1 and U1.

Unfortunately it doesn't seem to have made an improvement 

Are there any other common problem components I can look into?

 

[techknight]

you should run the lisa with the analog board exposed. then turn a can of compressed-air upside down and hit each component (after its warmed up) until you find the one that alleviates the symptoms.